On the surface of a windowpane for vehicles such as an automobile backlite, etc., there may be provided a conductor layer having a pattern such as a defogger for defogging, a deicer for deicing, an antenna for radio reception, etc. For example, a defogger or deicer is constituted of a heater line (strip part) and a bus bar connected to the heater line and having an electricity feeding point (terminal connection part) for feeding electricity into the heater line, and an antenna is constituted of an antenna conductor (strip part) and a terminal connection part for connecting an external antenna circuit to the antenna conductor.
A conductor layer is generally formed by printing a silver paste containing a silver powder and a glass frit in a prescribed pattern on the surface of a glass sheet, followed by sintering. Then, a metal-made connection terminal for connection to an external power source or antenna circuit is soldered onto the terminal connection part of the conductor layer. For soldering the connection terminal onto the terminal connection part, a tin-lead-silver based solder alloy has hitherto been used.
On the other hand, in the case of performing soldering on a glass sheet, there was encountered such a problem that cracking is easily caused on the glass sheet due to a thermal stress during heating and a residual stress after cooling generated following a temperature change of soldering. This is chiefly caused due to the fact that a difference in thermal expansion coefficient between the solder alloy and a soda lime glass which is widely used for automobile glasses is relatively large. Therefore, among tin-lead-silver based alloys, an alloy in which the content of lead having a low elastic modulus is made larger than that of tin having a high elastic modulus is used for soldering of a windowpane for vehicles. This is because the alloy having a larger content of lead is able to relieve the stresses generated in the glass sheet accompanied with a temperature change.
In recent years, the needs for lead-free products have been increasing in view of an increase of an interest in the environmental issue. Consequently, it is required to use a lead-free solder alloy, for example, a tin based lead-free solder alloy composed mainly of tin, for soldering the connection terminal onto the terminal connection part. However, there is a concern that the tin based lead-free solder alloy impairs the appearance of the conductor layer, and its joining strength is also insufficient. In addition, there is also involved such a problem that cracking is easily caused on the glass sheet.
As for these problems, Patent Document 1 proposes the use of a tin-silver based lead-free solder alloy composed mainly of tin and containing 1.5% by mass or more of silver.